Rubber-acrylic adhesive formulation

ABSTRACT

Pressure sensitive adhesive formulations of an acrylic polymer grafted with a hydrogenated rubber. In one embodiment, the polymer contains a hydroxyalkyl (meth)acrylate ester which is crosslinked with a titanium-containing chelated metal alkoxide. The adhesive formulations provide an exceptional combination of adhesion to low energy surfaces and high temperature cohesive strength.

This is a division of application Ser. No. 09/809,862, filed Mar. 16,2001.

FIELD OF THE INVENTION

The present invention relates to pressure sensitive adhesiveformulations. In particular, the invention relates to a pressuresensitive adhesive formulation comprising an acrylic polymer graftedwith an ethylene-butylene rubber macromer.

BACKGROUND OF THE INVENTION

Typical acrylic pressure sensitive adhesive formulations are copolymersof alkyl ester monomers, a functional monomer such as acrylic acid, andmay be crosslinked using, for example, aluminum chelates. Theseadhesives are generally deficient in adhesion to low energy surfaces.While adhesives may be tackified with rosin esters to improve lowsurface energy adhesion, tackification results in loss of heatresistance and poor aging properties. Even though good aging propertiesare compromised, tackified acrylic dispersions are sufficient for someapplications, e.g. most paper label uses and, indeed, have become thedominant paper label technology. These tackified acrylic adhesives,however, do not have sufficient resistance to degradation for mostgraphics and industrial tape applications in which acrylic solutions areconventionally used.

Rubber-resin formulations are often used to adhere to polyolefins andother low energy substrates. Typical compositions are natural rubber orstyrene block copolymers tackified With rosin esters. These formulationsprovide excellent tack and cohesive strength but discolor and lose tackon aging due to oxidative and UV light induced degradation. Formulationsof fully hydrogenated rubbers and resins, besides being more costly,generally do not have the required adhesive performance.

U.S. Pat. No. 5,625,005 discloses hybrid rubber-acrylic pressuresensitive adhesives described as having good UV resistance and agingcharacteristics along with high adhesion to non-polar surfaces. Despitethis advancement in the art, there remains a need for improved polymercompositions which can be used to prepare pressure sensitive adhesiveshaving sufficient adhesion and chemical resistance properties forapplications such as industrial tapes and transfer films, and exteriorgraphics applications on low energy, difficult to adhere surfaces. Thepresent invention addresses this need.

SUMMARY OF THE INVENTION

The invention provides adhesive formulations having outstanding coatingcharacteristics, adhesion to a wide variety of substrates, including lowenergy surfaces, while maintaining these performance properties athigher temperatures in their dried state.

One aspect of the invention is directed to a pressure-sensitive adhesivecomprising an acrylic polymer grafted with a rubber macromer. Preferredfor use is an ethylene-butylene macromer. In one embodiment, the acrylicpolymer comprises at least one low glass transition temperature (Tg)alkyl acrylate monomer containing from about 4 to about 18 carbon atomsin the alkyl group and at least one monomer having a high glasstransition temperature (i.e., a Tg greater than about 0° C.). Inpreferred embodiments of the invention the acrylic polymer may furthercomprise at least one hydroxy functional monomer and/or may alsocomprise at least one carboxy functional monomer. In a particularlypreferred embodiment, a crosslinking agent, such as an aluminum or atitanium crosslinking agent, is used.

Another aspect of the invention is directed to a pressure-sensitiveadhesive comprising an acrylic polymer comprising at least one low Tgalkyl acrylate monomer containing from about 4 to about 18 carbon atomsin the alkyl group grafted with a rubber macromer, preferably, anethylene-butylene macromer, the polymer being crosslinked using atitanium crosslinking agent. In a preferred embodiment, the acrylicpolymer comprises, in addition to an alkyl acrylate monomer, at leastone high Tg monomer, at least one hydroxy functional monomer and/or atleast one carboxy functional monomer. The use of a titanium-containingmetal alkoxide crosslinker has been discovered to impart excellent andunexpected high temperature performance.

Still another aspect of the invention is directed to a process of makinga pressure-sensitive adhesive comprising an acrylic polymer grafted witha rubber macromer, preferably an ethylene-butylene macromer, wherein themacromer is substantially free of metal or strong acid. Preferably, themolecular weight of the macromer used to make the adhesive ranges fromabout 2,000 to about 10,000. The process comprises reacting an acrylicpolymer component with a rubber macromer component, said macromercomponent being substantially free of catalyst used to prepare themacromer component.

Yet another aspect of the invention is directed adhesive articles, e.g.,industrial tapes, transfer films, and the like, comprising a pressuresensitive adhesive hybrid polymer. In one particularly preferredembodiment, the hybrid polymer comprises an ethylene-butylene macromer,2-ethylhexyl acrylate or similar low Tg acrylic monomer, methyl acrylateor similar high Tg monomer, and preferably a hydroxy functional monomersuch as hydroxyethyl acrylate.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the term “pressure-sensitive adhesive” refers to aviscoelastic material which adheres instantaneously to most substrateswith the application of slight pressure and remains permanently tacky. Apolymer is a pressure-sensitive adhesive within the meaning of the termas used herein if it has the properties of a pressure-sensitive adhesiveper se or functions as a pressure-sensitive adhesive by admixture withtackifiers, plasticizers or other additives.

The adhesive polymer of the invention is a rubber-acrylic hybrid polymercomprising an acrylic polymer backbone grafted with rubber macromersincluding, but not limited to, ethylene-butylene macromers,ethylene-propylene macromers and ethylene-butylene-propylene macromers.In general, the hybrid polymers are made by copolymerizing alkylacrylate ester monomers in the presence of a macromer containing areactive acrylic or methacrylic end group. This leads to a comb-typecopolymer having an acrylic backbone and side chains of macromer.

More specifically, acrylic polymer backbone contemplated for use in thepractice of the invention is formed of acrylate monomers of one or morelow Tg alkyl acrylates. Low transition temperature monomers are thosehaving a Tg of less than about 0° C. Preferred alkyl acrylates which maybe used to practice the invention have up to about 18 carbon atoms inthe alkyl group, preferably from about 4 to about 10 carbon atoms in thealkyl group. Alkyl acrylates for use in the invention include butylacrylate, amyl acrylate, hexyl acrylate, 2-ethylhexyl acrylate, isooctylacrylate, decyl acrylate, dodecyl acrylates, isomers thereof, andcombinations thereof. A preferred alkyl acrylate for use in the practiceof the invention is 2-ethyl hexyl acrylate.

The monomer system used to make the acrylic backbone polymer could besolely based on lowly Tg alkyl acrylate ester monomers, but ispreferably modified by inclusion of high Tg monomers and/or functionalcomonomers, in particular carboxy-containing functional monomers,and/or, even more preferably, hydroxy-containing functional monomers.

High Tg monomer components which may be present, and in some embodimentsare preferably present, include methyl acrylate, ethyl acrylate,isobutyl methacrylate, and/or vinyl acetate. The high Tg monomers may bepresent in a total amount of up to about 50% by weight, preferably fromabout 5 to about 50% by weight, even more preferably from about 10 toabout 40% by weight, based on total weight of the hybrid polymer.

The acrylic backbone polymer may also comprise one or more functionalmonomers. Preferred are carboxy and/or hydroxy functional monomers.

Carboxy functional monomers will typically be present in the hybridpolymer in an amount of up to about 7% by weight, more typically fromabout 1 to about 5% by weight, based on the total weight of themonomers. Useful carboxylic acids preferably contain from about 3 toabout 5 carbon atoms and include, among others, acrylic acid,methacrylic acid, itaconic acid, and the like. Acrylic acid, methacrylicacid and mixtures thereof are preferred.

In a particularly preferred embodiment, the acrylic backbone compriseshydroxy functional monomers such as hydroxyalkyl (meth)acrylate esters,and acrylic polymers used to form the backbone of the invention arepreferably acrylic ester/hydroxy (meth)alkyl ester copolymers. Specificexamples of hydroxy functional monomers include hydroxyethyl acrylate,hydroxypropyl acrylate, hydroxyethyl methacrylate and hydroxypropylmethacrylate. Hydroxy functional monomers are generally used in anamount of from about 1 to about 10%, preferably from about 3 to about7%.

Other comonomers can be used to modify the Tg of the acrylic polymer, tofurther enhance adhesion to various surfaces and/or to further enhancehigh temperature shear properties. Such comonomers include N-vinylpyrrolidone, N-vinyl caprolactam, N-alkyl (meth)acrylamides such ast-octyl acrylamide, cyanoethylacrylates, diacetoneacrylamide, N-vinylacetamide, N-vinyl formamide, glycidyl methacrylate and allyl glycidylether.

The monomer proportions of the acrylic polymer are adjusted in such away that the backbone polymer has a glass transition temperature of lessthan about −10° C., preferably from about −20° C. to about −60° C.

The macromers which may be used to prepare the graft copolymers have aglass transition temperature of about −30° C. or less, preferably about−50° C. to about −70° C., as determined by differential scanningcalorimetry (DSC), and are preferably present in an amount of from about5 to about 50 percent by weight of the hybrid polymer. Such macromersare commercially available from Kraton Polymers Company. While themolecular weight of the macromer can range from about 2,000 to about30,000, macromers for use in practicing the invention will preferablyhave a molecular weight range of from about 2,000 to about 10,000, asdetermined by gel permeation chromatography (GPC).

Conventionally, saturated rubber macromers may be prepared by a numberof well-known methods. One method involves an anionic polymerization toproduce a hydroxyl terminated conjugated diene polymer formed from, forexample, 1,3-butadiene and/or isoprene monomer, as described in U.S.Pat. No. 5,625,005, the disclosure of which is incorporated herein byreference. Reduction of at least 90%, preferably at least 95%, of theunsaturation in the low molecular weight monool can be achieved throughcatalytic hydrogenation as taught in U.S. Pat. Nos. Re. 27,145 and4,970,254, the disclosures of which are incorporated by referenceherein. Suitable saturated rubber monools are available from KratonPolymers Company. Kraton® L 1203 is a preferred grade. In the finalstep, the hydroxyl termination is reacted to form an acrylate ormethacrylate group by any of a number of well known methods. Theseinclude esterification or transesterification using a strong acid ormetal-containing catalyst, (e.g., compounds of Ti, Sn and the like), byreaction with an acid chloride, or via a urethane reaction employing ametal catalyst, as described in U.S. Pat. No. 5,625,005.

It has now been discovered that metal or acid residue present in themacromer used to prepare rubber acrylic hybrid polymer-based adhesives,in particular those comprising hydroxyl functional-containing polymers,can adversely affect adhesive properties. While certain low levels ofmetal or acid may be used for certain applications, it is preferablethat the macromers be substantially free of the catalyst used in thepolymerization thereof. Substantially free, as this term is definedherein, means that any catalyst residue remaining in the polymerizedmacromer, if any, will not cause problems in the preparation of thehybrid polymer. Removal of cataylst residue can be readily accomplishedusing methods well-known in the art.

The hybrid polymer of the invention may be prepared by conventionalpolymerization methods familiar to those of skill in the art. Thesemethods include, without limitation, solution polymerization, suspensionpolymerization and bulk polymerization. In solution, the graftcopolymers are synthesized by conventional free radical techniques usinga solvent mixture. The solvent blend, preferably ethyl acetate, hexaneand/or heptane, and toluene, imparts the solubility that is necessaryfor good coating behavior at low and high coat weights. In the practiceof the invention, it may also be advantageous to reduce the residualmonomer content following polymerization using methods which are knownand conventional in the art.

The preferred adhesive compositions are preferably crosslinked using achemical crosslinking agent. While the use of aluminum and titaniumcrosslinking agents may be used to practice the invention, it has beendiscovered that use of titanium containing metal alkoxide crosslinker isnecessary for high temperature performance, and is the preferredcrosslinker for hydroxyalkyl(meth)acrylate esters. The use of a titaniumcrosslinker imparts a yellowish color to the final product but, for manyapplications, is of little concern. The crosslinker is typically addedin an amount of from about 0.3% to about 2% by weight of the hybridpolymer.

The adhesive compositions of this invention are preferably tackified.The acrylic and rubber components of the hybrid polymer are believed toform a microphase separated structure in the solid state. Support forthis comes from the appearance of two distinct Tg's in the temperaturespectrum of viscoelastic properties corresponding to each component.Tackifying resins useful in these compositions are compatible with therubber macromer phase. Tackifiers compatible with the acrylic phase can,of course, be used with any acrylic polymer and the hybrid polymer ofthis invention is no exception. However, such tackifiers are typicallyderived from natural rosin and are associated with poor agingcharacteristics. It is an objective of this invention to overcome theseproblems. Thus the preferred tackifiers are synthetic hydrocarbon resinsderived from petroleum. Non-limiting examples of rubber phaseassociating resins include aliphatic olefin derived resins such as thoseavailable from Goodyear under the Wingtack® tradename and the Escorez®1300 series from Exxon. A common C₅ tackifying resin in this class is adiene-olefin copolymer of piperylene and 2-methyl-2-butene having asoftening point of about 95° C. This resin is available commerciallyunder the tradename Wingtack 95. The resins normally have ring and ballsoftening points as determined by ASTM method E28 between about 20° C.and 150° C. Also useful are C₉ aromatic/aliphatic olefin-derived resinsavailable from Exxon in the Escorez 2000 series. Hydrogenatedhydrocarbon resins are especially useful when the long term resistanceto oxidation and ultraviolet light exposure is required. Thesehydrogenated resins include such resins as the Escorez 5000 series ofhydrogenated cycloaliphatic resins from Exxon, hydrogenated C₉ and/or C₅resins such as Arkon® P series of resins by Arakawa Chemical,hydrogenated aromatic hydrocarbon; resins such as Regalrez 1018, 1085and the Regalite® R series of resins from Hercules Specialty Chemicals.Other useful resins include hydrogenated polyterpenes such as Clearon®P-105, P-115 and P-125 from the Yasuhara Yushi Kogyo Company of Japan.

The tackifying resin will normally be present at a level of 5 to 50% byweight of the adhesive composition and preferably at a level of about 10to 40% by weight of the adhesive composition.

The formulated adhesive may also include, excipients, diluents,emollients, plasticizers, antioxidants, anti-irritants, opacifiers,fillers, such as clay and silica, pigments and mixtures thereof,preservatives, as well as other components or additives.

The pressure sensitive adhesives of the invention may advantageously beused in the manufacture of adhesive articles including, but not limitedto, industrial tapes and transfer films. The adhesive articles areuseful over a wide temperature range, have improved UV resistance andadhere to a wide variety of substrates, including low energy surfaces,such as polyolefins, e.g., polyethylene and polypropylene, polyvinylfluoride, ethylene vinyl acetate, acetal, polystyrene, powder-coatedpaints, and the like. Single and double face tapes, as well as supportedand unsupported free films are encompassed by the invention. Alsoincluded, without limitation, are labels, decals, name plates,decorative and reflective materials, reclosable fasteners, theftprevention and anti-counterfeit devices.

In one embodiment, the adhesive article comprises an adhesive coated onat least one major surface of a backing having a first and second majorsurface. Useful backing substrates include, but are not limited to foam,metal, fabric, and various polymer films such as polypropylene,polyamide and polyester. The adhesive may be present on one or bothsurfaces of the backing. When the adhesive is coated on both surfaces ofthe backing, the adhesive on each surface can be the same or different.

EXAMPLES

In the following examples, the following adhesive test methods wereused.

Preparation of Coatings

The adhesive solutions were cast on a silicone coated release liner, airdried for 15 minutes, then dried for 3 minutes at 250° F. in a forcedair oven. The films were then laminated to a backing film andconditioned overnight at 22° C. and 50% relative humidity. Unlessotherwise indicated the dried adhesive film thickness was 1 mil (25microns) and the backing film was 2 mil polyester film.

Peel Adhesion

Peel adhesion at 180° between the backing and the adherend test panelwas measured according to Test Method number 1 of the Pressure SensitiveTape Council (PSTC), Northbrook, Ill., adapted as follows. The peelstrength was measured after wetting out a stainless steel (SS) panel for20 minutes or as otherwise indicated. The testing was also carried outon high density polyethylene (HDPE) panels. Unless otherwise indicated,all testing was performed at 22° C. and 50% relative humidity.

Shear Holding Power

Shear holding power was measured according to PSTC Test Method number 7,adapted as follows. The holding power was measured under a shear load of1 kg on a 0.5 inch by, 1 inch area, applied after wetting out the testpanel for 15 minutes. The testing was also carried out on high densitypolyethylene (HDPE) panels. Unless otherwise indicated, all testing wasperformed at 22° C. and 50% relative humidity.

Shear Adhesion Failure Temperature (SAFT)

The SAFT measurement was performed by placing a 1 inch by 1 inch bondedtest specimen in an oven at 140° F. under a shear load of 1 kg. (15minutes wet out at room temperature was allowed before applying theload.) The oven temperature was then raised in 10° F. increments every10 minutes and the temperature at which the bond failed was recorded.

Example 1

This example describes the preparation of a hybrid polymer solutionusing a methacrylate terminated macromer which was substantially free ofstrong acid or metal, catalysts. The molecular weight averages of themacromer were determined by GPC, relative to polystyrene standards, tobe Mn=6600, Mw=7200 Daltons.

An initial charge-mixture containing 59.73 g 2-ethylhexyl acrylate(2-EHA), 22.72 g ethylene-butylene macromer, 17.05 g methyl acrylate(MA), 5.67g 2-hydroxyethylacrylate (2-HEA), 55.0 g ethyl acetate(EtOAc), 64.19 g hexanes (a standard mixed isomer grade), and 0.28 gazobis(isobutyronitrile) (AIBN) was prepared and charged to a 3 liter4-neck round bottomed flask equipped with stainless steel stirrer,thermometer, condenser, water bath, and slow addition funnels. Theinitial charge was heated to reflux while stirring. After 10 minutes atreflux a monomer mix containing 229.0 g 2-EHA, 128.59 gethylene-butylene macromer, 65.45 g MA, 21.84 g 2-HEA, 27.5 g hexanesand an initiator mix containing 236.5 g EtOAc, 55.0 g hexanes, 1.38 gAIBN were simultaneously, separately, and uniformly added over a periodof 2 hours and 3 hours, respectively. At the end of the additions theflask-contents were held at reflux for a further 2 hours. Next theresidual monomers were scavenged using a short half-life initiator addedover a one hour period and the solution was held under reflux for afurther hour. Then diluent consisting of 183.3 g toluene was slowlyadded to the reactor contents while cooling the contents to roomtemperature. The polymer solution maintained a fluid viscositythroughout the reaction and showed no tendency to climb the reactorstirring shaft.

The polymer solution had a solids content of 42.7% and a Brookfieldviscosity of 2500 mPa·s. The molecular weight averages, determined bygel permeation chromatography, were Mw=560,000 and Mn=34,000.

Examples 2 and 3

Since it has been discovered that use of strong acid catalyst or metalcatalyst used to prepare the macromer has adverse effects, experimentswere conducted to study the effect of strong acid catalysts on thepolymerization process (Example 2), and to study the effect of metalcatalysts on the polymerization process (Example 3).

Example 2

A series of polymers was prepared according to the procedure describedin Example 1. The macromer used to prepare this series containedp-toluenesulfonic acid (p-TSA) at a range of concentrations. The adverseeffect of higher levels of p-TSA is described in Table 1. At the higherlevels of acid the in-process solution viscosity increased dramaticallydue to network formation in the polymer. This ultimately led to gelformation and an unusable product. The problem is first manifested as atendency for the solution to climb the stirring shaft. This phenomenonis known as the Weissehberg effect. It results in poor mixing andincreases the torque with the result that additional power is requiredto maintain stirring. In an extreme case it can interfere with the shaftbearings and seal. The polymer which results, even if not gelled,exhibits poor, flow and leveling during the adhesive coating process.

TABLE 1 p-TSA (ppm) 75 150 300 500 Observation Fluid Fluid IncreasedHigh viscosity viscosity. viscosity. viscosity but still leading to gelNo shaft Minimal fluid. Moderate formation. climbing. shaft shaftclimbing. climbing.

Example 3

Polymer solutions were prepared according to Example 1. The macromerused to prepare these polymers contained tin at a range ofconcentrations. The adverse effect of higher levels of tin is describedin Table 2.

TABLE 2 Sn (ppm) 100 150 300 Observation Fluid viscosity. Increasedviscosity. High viscosity. No shaft climbing. Significant shaft Severeshaft climbing. climbing. Gel formation.

Example 4

Polymer solutions were prepared according to Example 1, substituting anequimolar concentration of hydroxypropyl methacrylate for HEA monomer.The macromer used to prepare these polymers contained tin at a range ofconcentrations, as provided in Table 3. The results show that a higherlevel of tin can be tolerated with this monomer without causing problemsin the polymerization process.

TABLE 3 Sn (ppm) 120 230 Observation Fluid viscosity. Slightly increasedNo shaft climbing. Viscosity but no shaft climbing.

Example 5

This example describes the preparation of polymers, with and without ahigh Tg acrylic comonomer. Two series of hybrid polymers were preparedfollowing the procedure in Example 1 with the exception that the diluentsolvent was ethyl acetate. In the second series acrylic acid (AA) wassubstituted for the HEA. The experimental design explored the use ofhigh Tg monomer, the level of macromer, and the effects of hydroxylversus carboxyl functionality. Results are shown in Table 4.

TABLE 4 A B C D E F G H I J Composition % 2-EHA 80 55 25 50 52.5 80 5525 50 52.5 % EB rubber 15 40 40 15 27.5 15 40 40 15 27.5 % MA 0 0 30 3015 0 0 30 30 15 % AA 5 5 5 5 5 0 0 0 0 0 % HEA 0 0 0 0 0 5 5 5 5 5Properties Solids, % 42.6 42.1 38.6 40.1 40.8 42.5 42.1 39.6 40.2 42.6Viscosity, Pa · s 1.6 2.4 13 >200 12 9.0 1.1 8.2 16.2 13.3 Mw × 10⁻⁵ 3.23.9 15 5.2 5.2 3.5 3.2 4.7 5.0 4.2 Mn × 10⁻⁴ 2.6 2.5 2.8 3.2 3.1 2.5 2.42.4 3.0 2.8

All compositions in the design could be prepared by this procedurealthough in one case, polymer solution D, the viscosity was very high.To help control viscosity, dilution with a hydrocarbon or aromatichydrocarbon solvent, as used in Example 1, is preferred instead of ethylacetate. The molecular weights, as determined by GPC, are all high.

Example 6

This example describes the adhesive performance testing of the polymersprepared in Example 5. The samples were tested following the proceduresdescribed above. The results are given in Table 5.

TABLE 5 A B C D E F G H I J Adhesion to SS Peel, oz/in. 50 32 48 50 5715 17 27 33 41 Shear, hours 0.3 0.4 41 154 6 0 0 2 55 1 Adhesion to HDPEPeel, oz/in. 17 13 10 8 9 16 17 8 3 14 Shear, hours 0.2 0.2 13 96 5 0 02 28 1

The results demonstrate that in the absence of the high Tg monomer,methyl acrylate, very poor shear holding power is obtained.

Example 7

This example shows the effect of adding a titanium crosslinking agent tothe polymer solutions of Example 5. Samples D and I were omitted sincethey already exhibit good cohesive strength. A solution consisting of12.3 g isopropyl alcohol, 15.3 g 2,4-pentanedione and 2.4 g Tyzor GBA(available from the Du Pont Company, Wilmington, Del.) was prepared.Tyzor GBA is a 75% solution in alcohols of bis (2,4-pentanedionate-O,O′)bis (2-propanolato)-titanium. The crosslinker solution was stirred intothe polymer solutions at the indicated concentrations (weight on weightof polymer) and the adhesive performance was measured. The results areshown in Tables 6 and 7.

TABLE 6 A B C E Active crosslinker, % 0.375 0.375 0.075 0.15 Adhesion toSS Peel, oz/in. 47 31 46 62 Shear, hours 5 2 41 34 Adhesion to HDPEPeel, oz/in. 13 15 14 7 Shear, hours 3.5 1.5 21 20

TABLE 7 F G H J Active crosslinker, % 0.9 0.9 0.6 0.6 Adhesion to SSPeel, oz/in. 5 6 25 22 Shear, hours 5 3 55 86 Adhesion to HDPE Peel,oz/in. 2 2 7 6 Shear, hours 3 2 24 31

This demonstrates that the titanium crosslinking agent effectivelyincreases the cohesive strength but that for a high cohesive strength itis necessary to have the high Tg monomer present in the polymercomposition (compare samples C and E with A and B, and samples H and Jwith F and G). While a trade off between adhesion and cohesion is to beexpected upon addition of a crosslinker, one sees that the peel strengthof the hydroxy-functional polymers F and G, even on stainless steel, isseverely reduced.

Example 8

This example shows the compatibility of hydrocarbon tackifying resinswith the polymers of Example 5. The tackifying resins were dissolved inthe polymer solutions at a high loading (60 parts tackifier to 100 partspolymer on a dry weight basis). The solutions were cast on glass plates,dried, and visually inspected for clarity. Those that were clear werejudged to be compatible. The tackifiers used in this experiment wereWingtack® 95, a C₅ resin available from the Goodyear Company, andEscorez® 2596, an aromatic/aliphatic resin available from ExxonMobile.The results, denoted by C for compatible and I for incompatible, areshown in Table 8.

TABLE 8 Polymer Wingtack 95 Escorez 2596 A I I B I I C I I D I I E C C FI I G I I H C C I C C J C C

These results show that the less polar hydroxy-functional polymers havebroader compatibility than the carboxy-functional materials. They alsounexpectedly show that the hydroxy-functional polymers which contain thehigh Tg monomer, methyl acrylate, i.e samples H, I and J, are compatiblewhereas those without, namely F and G, are incompatible.

Example 9

This example shows the effect on adhesive performance of formulatingwith a C₅ aliphatic hydrocarbon tackifying resin. A series of mixtureswere prepared with increasing level of tackifier. To 100 parts ofpolymer J, on a dry weight basis, 10, 20 and 40 parts of Wingtack 95,0.8 parts of Tyzor GBA and 0.5 parts of Irganox® 1010 (an antioxidantsold by Ciba Specialty Chemicals) were mixed in solution. The adhesiveperformance was measured according to the methods in Example 6. Acomparison is made with two acrylic polymers, DURO-TAK® 72-8746(designated A) and DURO-TAK 80-1105 (designated B), sold by NationalStarch and Chemical Company, and formulated with 15 and 40 parts perhundred polymer of rosin ester tackifier, respectively. The results areshown in Table 9.

TABLE 9 Acrylic Acrylic Tackifier A B Hybrid Polymer J concentration, wt% 15 40 0 10 20 40 Adhesion to SS Peel, oz/in. 58 66 22 32 48 94 Shear,hours 146 4 86 >142 62 38 SAFT, ° F. 165 165 n.m. n.m. n.m. 200 Adhesionto HDPE Peel, oz/in. 14 38 6 18 27 55 Shear, hours 82 4 31 15 18 13 n.m.= not measured

The results show that the hybrid polymer has an excellent response totackifier, giving substantial increase in peel strength on stainlesssteel and especially on the low surface energy substrate HDPE. Thehybrid psa of this invention has much higher cohesive strength than theacrylic at high tackifier loading and therefore provides a superiorbalance of properties. Unexpectedly, the heat resistance, as measured bythe SAFT, is significantly higher for the hybrid psa, outperforming ahigh cohesion acrylic with a much lower tackifier loading.

Example 10

This example shows the effect on adhesive performance of formulatingwith hydrogenated cycloaliphatic hydrocarbon tackifying resins. Mixtureswere prepared as described in Example 8 using 40 parts of Escorez 5415and 40 parts of Escorez 5600 on 100 parts of polymer J. The crosslinkerwas Tyzor GBA at 1.2 parts per hundred polymer. Escorez 5415 is ahydrogenated cycloaliphatic resin with a Ring and Ball softening pointof 118° C. Escorez 5600 is a hydrogenated aromatic modifiedcycloaliphatic resin with a Ring and Ball softening point of 103° C.Both adhesives gave a clear film indicating compatibility with thepolymer. The adhesive performance was measured according to the methodsin Example 6 using a 2 mil thick coating of adhesive. The results areshown in Table 10. For comparison, results are shown for Wingtack 95under same formulation parameters and test conditions.

TABLE 10 Escorez Escorez Wingtack 95 5415 5600 Adhesion to SS Peel,oz/in. 137 70 78 SAFT, ° F. 230 255 290 Adhesion to HDPE Peel, oz/in. 6252 42

The results show that the hybrid polymer is compatible with hydrogenatedcycloaliphatic resins and that these resins impart additional heatresistance with only slight sacrifice in peel strength on the low energysubstrate, HDPE.

Example 11

This example compares a titanium with an aluminum crosslinking agent. Apolymer solution was prepared according to Example 1 and formulated with40% Wingtack 95 and 0.8% Tyzor GBA (75% active) expressed as weight onweight of dry polymer. A second solution was similarly prepared exceptthat the Tyzor was substituted with an equivalent concentration (0.7%)of aluminum tris(acetyl acetonate). The adhesives were then coated andtested according to the procedures described above. The results areshown in Table 11.

TABLE 11 Crosslinking agent Tyzor GBA Al (ac.ac.)₃ Active concentration,wt % 0.6 0.7 1.0 1.2 Adhesion to SS Peel, oz/in. 91 120 104 83 Shear,hours 34.8 4.0 4.3 5.0

The tests showed that the aluminum crosslinking agent when compared totitanium is ineffective in developing cohesive strength. To confirm thesuperiority of a titanium crosslinking agent in the present invention,two further samples with an increased concentration of aluminum wereprepared and tested. The results, given in the above Table, show thatincreasing the level of aluminum has little positive effect on shearstrength but does reduce the peel adhesion.

Example 12

This example shows the effect of having both hydroxyl and carboxylfunctionality in the polymer. A polymer solution was prepared by themethod of Example 1 substituting 4 wt % HEA and 1 wt % M for the 5 wt %HEA found in the Example polymer. This is designated as polymer K. Asecond polymer solution, designated L, was prepared with 0.15 wt %glycidyl methacrylate as an additional component. The adhesiveperformance of these polymers, tested without additional formulation atdry coating thickness of 2 mils, is shown in Table 12.

TABLE 12 Polymer K Polymer L Adhesion to SS Peel, oz/in. 74 94 Shear,hours 25 22 SAFT, ° F. 150 150 Adhesion to HDPE Peel, oz/in. 38 41

Both polymers K and L showed excellent performance for an unformulatedbase polymer. In particular, the high cohesion and heat resistance giveswider formulating latitude to accept additional tackifier and/or reducethe level of crosslinker. See Example 13.

Example 13

This example shows that the a polymer with combined hydroxyl andcarboxyl functionality can be formulated to give substantially improvedheat resistance while retaining a high level of peel adhesion. Theexample further demonstrates that an aluminum crosslinking agent canalso be used in this case. The polymer solutions were formulated withWingtack 95 tackifying resin and crosslinker as shown in Table 13. Theweights are expressed on weight of polymers solids. The adhesive coatingthickness was 2 mils.

TABLE 13 Polymer Example 1 L L Crosslinking agent Tyzor GBA Tyzor GBA Al(ac.ac.)₃ Active concentration, wt % 0.6 0.45 0.41 Tackifier, wt% 40 5050 Adhesion to SS Peel, oz/in. 140 152 149 SAFT, ° F. 240 >300 280Adhesion to HDPE Peel, oz/in. 64 66 63

These results show that a very substantial increase in the heatresistance has been achieved. They further show that an aluminumcrosslinking agent can be used with polymer L with some sacrifice inheat resistance but still a good balance of properties. This is to becompared with the poor result obtained when attempting to crosslink thehydroxy-functional polymer of Example 1 with aluminum (see Example 11).The use of aluminum may be desirable in applications where absence ofcolor is important.

Many modifications and variations of this invention can be made withoutdeparting from its spirit and scope, as will be apparent to thoseskilled in the art. The specific embodiments described herein areoffered by way of example only, and the invention is to be limited onlyby the terms of the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. A process of making a pressure-sensitiveadhesive, said process comprising reacting an acrylic polymer componentwith a rubber macromer component wherein said acrylic polymer comprisesat least one alkyl acrylate monomer containing from, about 4 to about 18carbon atoms in the alkyl group and at least one monomer whosehomopolymer has a glass transition temperature greater than about 0° C.,and said macromer has a glass transition temperature of about −30° C. orless, said macromer component being substanially free of catalyst usedto prepare the macromer component.
 2. The process of claim 1 wherein themacromer comprises poly(ethylene-butylene), poly(ethylene-propylene) orpoly(ethylene-butylene-propylene).
 3. The process of claim 1 wherein themacromer is substanially free of metal containing catalyst.
 4. Theprocess of claim 1 wherein the macromer is substantially free of strongacid catalyst.
 5. The process of claim 1 wherein the macromer has amolecular weight of from about 2,000 to about 10,000.
 6. The process ofclaim 1 wherein the macromer has a glass transition temperature of fromabout −50° C. to about −70° C.
 7. A article of manufacture comprising apressure-sensitive adhesive, said adhesive comprising an acrylic polymercopolymerized with a rubber macromer, wherein the polymer comprises atleast one alkyl acrylate monomer containing from about 4 to about 18carbon atoms in the alkyl group and at least one monomer whosehomopolymer has a transition temperature greater than about 0° C., andwherein the macromer has a glass transition temperature of about −30° C.or less.
 8. The article of claim 7 which is a pressure-sensitiveadhesive tape.
 9. An article of manufacture comprising apressure-sensitive adhesive, said adhesive comprising an acrylic polymercopolymerized with a rubber macromer, the polymer comprising at leastone alkyl acrylate monomer containing from about 4 to about 18 carbonatoms in the alkyl group, and wherein the polymer is crosslinked using atitanium crosslinking agent.
 10. The article of claim 9 which is apressure sensitive adhesive tape.